engleski

UV photoconductivity of nanocrystalline TiO2 films annealed in oxygen

The effects of annealing in reactive gases (H2, N2, O2) upon the optoelectric properties of nanophased titanium dioxide (TiO2) prepared by a chemical vapour deposition (CVD) were investigated. Nanocrystalline structure with nanosize grains and pores was determined by Grazing-incidence small-angle scattering of synchrotron radiation (GISAXS). The annealing (up to 1073K) in H2 and N2 generally proved detrimental to photoconductivity due to the overall increase of the electrical conductivity of the samples. In this work the results of UV (248-404 nm) photoconductivity measurements on TiO2 films annealed in O2 at 773K and 1073K are presented. A rather long illumination times (2 hours typically) allowed to clearly distinguish two types of nonequilibrium photoconductivity variation with time. A fast exponential photoconductivity increase occured during the initial stage of irradiations, while a slow power-type increase was observed in the later stage. A nonlinear combination of both functions was used in numerical fitting procedure, which allowed to precisely determine the asymptotic value of exponential photoconductivity increase. A relative quantum efficiency for both as-prepared and annealed samples exhibits a non-monotonous variation with photon energy. Such wavelength-dependence variation might be due to the electronic density function structure at the valence-band edge, or near-valence-band levels in the gap. Generally, tha samples annealed at higher temperature exhibit higher quantum efficiency and shorter time constants of teh excitation processes in the examined UV range.

TiO2 thin films; nanocrystalline-materials-structure; UV photoconductivity; GISAX

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